Skin-like pressure and strain sensors based ontransparent elastic films of carbon nanotubesDarren J. Lipomi1†, Michael Vosgueritchian1†, Benjamin C-K. Tee2†, Sondra L. Hellstrom3,Jennifer A. Lee1, Courtney H. Fox1 and Zhenan Bao1*
Transparent, elastic conductors are essential components ofelectronic and optoelectronic devices that facilitate humaninteraction and biofeedback, such as interactive electronics1,implantable medical devices2 and robotic systems withhuman-like sensing capabilities3. The availability of conductingthin films with these properties could lead to the developmentof skin-like sensors4 that stretch reversibly, sense pressure(not just touch), bend into hairpin turns, integrate with collap-sible, stretchable and mechanically robust displays5 and solarcells6, and also wrap around non-planar and biological7–9 sur-faces such as skin10 and organs11, without wrinkling. Wereport transparent, conducting spray-deposited films ofsingle-walled carbon nanotubes that can be rendered stretch-able by applying strain along each axis, and then releasingthis strain. This process produces spring-like structures in thenanotubes that accommodate strains of up to 150% anddemonstrate conductivities as high as 2,200 S cm21 in thestretched state. We also use the nanotube films as electrodesin arrays of transparent, stretchable capacitors, which behaveas pressure and strain sensors.
Metallic films on elastomeric substrates can accommodate strainby means of controlled fracture12 or buckling13, but they are gener-ally opaque. Conductive polymers can be buckled to form stretch-able transparent electrodes6,14, but topographic buckles may beincompatible with devices that require planar interfaces. Films ofcarbon nanotubes15 and graphene16 are candidates for stretchable,transparent electrodes because (1) the long mean-free path of elec-trons in defect-free films produces high conductivity, withoutdecreasing the transparency17, and (2) networks of nanotubes andgraphene sheets permit some elasticity without destroying the con-tiguity of the film. One collaboration16,18 has produced graphenesheets with values of transparency T and sheet resistance Rsapproaching those of tin-doped indium oxide (ITO), but the resist-ance increased by an order of magnitude when strained by 30%(ref. 16). Others have demonstrated uniaxial stretchability inhighly aligned films of nanotubes pulled from vertical forests19.There have also been reports of films of nanotubes that can bestretched by up to 100% along the axis of aligned nanotubeswithout a significant change in resistance15. Randomly depositedfilms of nanotubes have been stretched up to 700%, but the resis-tance increased by an order of magnitude following the applicationof !50% strain20. Recently, researchers reported a transparentnanotube film embedded in an elastomer for stretchable organiclight-emitting devices; the resistance of the most conductive film(50 V sq21 at T! 63%) increased by 100% at 50% strain21.Stretchable, opaque networks of conductive particles demonstratedthus far include a nanotube–fluoroelastomer composite with
conductivity of 9.7 S cm21 at 118% strain5, a nanotube–silvercomposite material with 20 S cm21 at 140% (ref. 22) and bucklednanotube films with 900 S cm21 at 40% (ref. 23). Combining highconductivity (s . 100 S cm21) and transparency (.80%) at highstrain (1" 150%) remains a challenge.
We produced conductive, transparent, stretchable nanotubefilms by spray-coating (nanotube length! 2–3 mm)24,25 directlyonto a substrate of poly(dimethylsiloxane) (PDMS, activated byexposure to ultraviolet/O3) from a solution in N-methylpyrroli-done. We obtained the best values of Rs and s by spin-coating asolution of charge-transfer dopant (tetrafluorotetracyanoquinodi-methane (F4TCNQ) in chloroform) over the films26. Doped andundoped films exhibited similar electromechanical behaviour. Weobtained values of Rs! 328 V sq21 and T! 79%, and maximumvalues of s! 1,100 S cm21 for a 100 nm film with T! 68%, at0% strain. (See Supplementary Fig. S1 for the measurement offilm thickness.)
Figure 1a presents the evolution of the change in resistance(DR/R0) as a function of strain for seven stages of applied strainand relaxation: 0 # 50% # 0% # 100% # 0% # 150% # 0%# 200%. With the first application of 50% strain, R increased by0.71. We attribute this increase to irrecoverable loss of junctionsbetween nanotubes. When we returned the film to 0% strain,DR/R0 decreased to 0.64 (as opposed to 0, its original value).Following the application of 100% strain, the resistance retraceditself until it reached 50% (the previous maximum strain), afterwhich the slope of DR/R0 increased. We observed similar behaviourwhen we relaxed the film on reaching 1! 100% and 150%. At 1$170%, the resistance of the film increased irreversibly by severalorders of magnitude. We estimate a lower limit on conductivity at150% strain of 2,200 S cm21 (see Supplementary Information fora discussion of conductivity under strain).
The effect of strain history on resistance implies that these nano-tube films can be ‘programmed’ by the first cycle of strain andrelease, to be reversibly stretchable within the range defined by thefirst strain. Figure 1b shows four cycles of strain from 0 to 50%, inwhich the resistance increases reversibly by !10% because thefilm was previously strained to a maximum of 50%. Measurementof R over the course of 12,500 cycles of stretching produced theplot shown in Fig. 1c. Over the course of this experiment, the resis-tance had decreased by 22% at the 1,500th cycle, and then increasedlinearly. We observed the same minimum in resistance at %1,000cycles of stretching in three similar experiments. We attribute theminimum in resistance to a period in which the nanotubebundles adopted their optimum morphology. Subsequent cycles ofstretching possibly decreased the number of conductive junctionsbetween bundles.
1Department of Chemical Engineering, Stanford University, Stanford, California 94305, USA, 2Department of Electrical Engineering, Stanford University,Stanford, California 94305, USA, 3Department of Applied Physics, Stanford University, Stanford, California 94305, USA; †These authors contributed equallyto this work. *e-mail: [email protected]
LETTERSPUBLISHED ONLINE: 23 OCTOBER 2011 | DOI: 10.1038/NNANO.2011.184
NATURE NANOTECHNOLOGY | VOL 6 | DECEMBER 2011 | www.nature.com/naturenanotechnology788
We examined the morphology of the nanotube films usingatomic force microscopy (AFM) to understand why the resistanceof the film was a function of its strain history. Figure 2 shows aseries of schematic diagrams depicting the change in morphologyof nanotubes on a PDMS substrate with strain, as well as corre-sponding AFM images. The ‘as-deposited’ film (Fig. 2a) exhibitedbundles (diameter, 10–20 nm) of nanotubes with isotropic orien-tations. Activation of the surface with ultraviolet/O3 before depo-sition was critical, because the nanotube bundles formed large,sparse aggregates non-uniformly on hydrophobic substrates.Figure 2b shows an AFM image of the nanotube film understrain. The application of strain exerted tensile stress on bundleswith components oriented with the axis of strain and alignedthem to it (Fig. 2b, dashed box). Compressive stress (due to thePoisson effect) on bundles oriented perpendicular to the axis ofstrain caused them to buckle in plane into waves (Fig. 2b, solidbox). After stretching the film for the first time, relaxation to 0%strain produced waves in the bundles that had been aligned bystretching (Fig. 2c). The amplitude of waves increased with theinitial strain (Supplementary Fig. S2). Other researchers have
observed a different, although similar, phenomenon, with the buck-ling of individual nanotubes (as opposed to bundles) on the surfaceof PDMS under small compressive strains of 5% (as opposed to150%), with buckling amplitudes ,10 nm and perpendicular (asopposed to parallel) to the plane of the substrate27.
The stretch-induced change in the morphology of the uniaxiallystretched films produced unequal conductivities along the stretchedand unstretched axes. When both axes were stretched and released,all nanotube bundles exhibited buckling, but the orientations wererandom (Fig. 2d), and the resistance was the same along the stretchedand unstretched axes. Biaxially stretched films were reversibly stretch-able in any direction (see Supplementary Information and Fig. S3).
The technological goal is to integrate these stretchable, trans-parent conductors into interactive optoelectronic devices andsensors for biofeedback. We therefore fabricated transparent andstretchable parallel-plate capacitors that could manifest changes inpressure and strain as changes in capacitance (Fig. 3a). Thedevices comprised two strips of PDMS bearing stretchable nanotubefilms, which we laminated together, face-to-face, with Ecoflexsilicone elastomer. Ecoflex (Shore hardness 00-10) is more easily
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Figure 1 | Effects of applied strain on films of spray-coated carbonnanotubes on PDMS substrates. a, Change in resistance DR/R0 versusstrain 1 for a nanotube film on a PDMS substrate. When the film is strained(arrow, bottom left), DR/R0 increases, and remains constant as the strain isreleased. When the strain is increased again, DR/R0 remains constant, andthen increases when 1 exceeds the value at which the strain was releasedbefore. This sequence is repeated up to DR/R0 $ 5 and 1$ 150%. b, DR/R0
versus time in response to four cycles of stretching from 0 to 50%.c, Resistance versus number of stretches (on a log scale) over 12,500cycles of stretching to 25%.
Compression
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Figure 2 | Evolution of morphology of films of carbon nanotubes withstretching. Schematics (left) and corresponding AFM phase images (right)of nanotube films as deposited (a), under strain (b), stretched and releasedalong one axis (c), and stretched and released along two axes (d).The bundles are considerably longer than the individual nanotubeswithin them. Dashed and solid white boxes highlight the bundles ofnanotubes buckled along the horizontal and vertical axes, respectively.Scale bars, 600 nm.
deformed than PDMS (Shore hardness A-48)28. The capacitance ofa parallel-plate capacitor is proportional to 1/d, where d is thespacing between plates. Application of pressure (Fig. 3a (left), b,d)and tensile strain (Fig. 3a (right), c, e) both resulted in a shorteneddistance between the electrodes (d&). Capacitance C is linearlydependent on pressure to 1 MPa (Fig. 3b) and strain to 50%(Fig. 3c) over the ranges tested. The smallest change in capacitancedistinguishable from noise was %50 kPa. The figure of merit of con-ventional strain gauges is the gauge factor, (DR/R0)/1 (ref. 29). Wecan also define a capacitive gauge factor, (DC/C0)/1, which is theslope of the linear fit in Fig. 3c (see Supplementary Informationfor discussion); in this case we determined (DC/C0)/1 to be0.004. Figure 3d shows capacitance versus time for four cycles ofapplied pressure using an electrically insulating tip to apply theload. Figure 3e shows a similar plot of capacitance versus timeover four cycles of stretching to 30%. The timescale over whichthe pixels recovered was smaller than that over which our instru-ments could load and unload the sample, !125 ms.
We next fabricated a grid of capacitors to produce a device thathad spatial resolution (Fig. 4). We began by depositing nanotubelines through a PDMS membrane that contained apertures (step 1)30.Applying strain rendered the film reversibly stretchable (step 2).We positioned a second patterned substrate orthogonal to the first(step 3), placed liquid eutectic gallium-indium (EGaIn)28,31,32 andcopper wires at the ends of the nanotube lines, and laminated thesubstrates together with Ecoflex (step 4).
We formed patterns of nanotube films with linewidths of 0.6–2 mm and pitches of 2–4 mm. We generated arrays of 4–64 capaci-tors (‘pixels’) with areas of 0.4–4 mm2 and pitches of 2–4 mm. Thethickness of the Ecoflex layer was %300 mm. Figure 5a,b shows thelargest array we fabricated: an 8 ' 8 array of nanotube lines, withwidth and spacing of 2 mm. The average capacitance of each pixelwas 13.3+1.4 pF (N! 64). Transparency of the nanotube linesvaried across the substrate from 88 to 95%.
In principle, changes in capacitance due to strain could be dis-tinguished from those due to pressure. Tensile straining wouldaffect pixels along the axis of strain; pressure would affect thepixels in the immediate vicinity of the load. We found that thecrosstalk between adjacent pixels in the 64-pixel device was low,and the change in capacitance registered by the pixel on whichpressure was applied was five times higher than the average ofthat registered by the four adjacent pixels (Fig. 5c). This obser-vation highlights the advantages of using stretchable materialsfor pressure sensors, for which the greatest compression occursat the site of the load. The crosstalk for devices fabricated on a
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Figure 3 | Use of stretchable nanotube films in compressible capacitors that can sense pressure and strain. a, Schematic showing a stretchable capacitorwith transparent electrode (top), and the same capacitor after being placed under pressure (left) and being stretched (right). b,c, Change in capacitanceDC/C0 versus pressure P (b) and strain 1 (e). d,e, DC/C0 versus time t over four cycles of applied pressure (d) and stretching (e).
Step 1
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Figure 4 | Summary of processes used to fabricate arrays of transparent,compressible, capacitive sensors. Spray-coating through a stencil maskproduces lines of randomly oriented nanotubes (step 1). A one-timeapplication of strain and release produces waves in the direction of strain(step 2). A second patterned substrate is positioned (face to face) over thefirst (step 3). The two substrates are bonded together using Ecoflex siliconeelastomer, which, when cured, serves as a compressible dielectric layer(step 4). Drops of a liquid metal, EGaIn, make conformal contact with thetermini of the nanotube electrodes and are embedded within the device.Copper nanowires connect the device to an LCR meter in the laboratory.
non-stretchable polyester substrate was approximately two timesgreater than that of the present device, as determined by the rela-tive increase in capacitance measured in pixels 4 mm from the siteof the load33.
Our device is less sensitive than another ‘skin-like’ devicereported in the literature33, but there are currently no other
devices that are both transparent and stretchable, and few havedemonstrated the ability to detect both pressure and strain4. Onedevice has been demonstrated based on nanowire field-effecttransistors that could detect a few kilopascals34, whereas anotherdevice with the geometry of a fishnet could undergo tensile defor-mations while detecting pressures on the order of 10 kPa (ref. 35).The pressures detectable by our devices, %50 kPa, correspondroughly to that of a firm pinch by opposing fingers. The architectureof the device, however, was not optimized.
Our devices were monolithically integrated, extremely mechani-cally compliant, physically robust and easily fabricated. The stretch-able, transparent nanotube electrodes were prepared withoutdispersion in an elastic matrix, without pre-straining the substrate,and patterned simply using stencil masks. In the future, it shouldbe possible to use these materials and principles to designorganic, skin-like devices with other human—and superhuman—characteristics36, such as the abilities to sense moisture, temperature,light6 and chemical and biological species37.
MethodsPreparation of substrates. PDMS (Dow Corning Sylgard 184; ratio of base tocrosslinker, 10:1 by mass) was mixed, degassed and poured against the polishedsurface of a silicon wafer bearing either a 300 nm thermal oxide or native oxidelayer. Before first use, the surfaces of the wafers were activated with oxygenplasma (150 W, 60 s, 400 mtorr) and passivated with the vapours of(tridecafluoro-1,1,2,2-tetrahydrooctyl)-1-trichlorosilane in a vacuum desiccator for"4 h. Curing in an oven at 60 8C for 2 h produced PDMS membranes that were%0.3 mm thick. These membranes were cut into squares or rectangles with a razorblade with lengths and widths of 2–8 cm.
Preparation of carbon nanotube solution. Arc-discharge single-walled nanotubes(Hanwaha Nanotech Corp.) were ultrasonicated in N-methylpyrrolidone at 30%power for 30 min. The solution was then centrifuged for 30 min at 8,000 r.p.m. toremove large bundles, amorphous carbon or other contaminants. The top 75% ofthe solution was decanted for spray coating. The final solution had a concentrationof %150 mg ml21.
Spray coatings and dependence of contact angle on strain. Nanotubes were spray-coated using a commercial airbrush (Master Airbrush, Model SB844-SET). ThePDMS substrates were first activated with ultraviolet/ozone for 20 min, then held at180 8C on a hotplate, and the nanotubes were sprayed at a distance of %10 cm usingan airbrush pressure of 35 psi. We used a laser-cut PDMS membrane that had long,parallel rectangular apertures (to produce parallel lines) as a stencil mask. Multiplepasses of the airbrush (.100) were performed until the desired transparency wasreached. The patterned substrates were placed in a vacuum oven at 100 8C for 1 h toremove residual solvent.
We found that the surface of the ultraviolet/ozone-treated PDMS substratesbecame more hydrophobic with strain: when stretching from 0 to 60%, the watercontact angle increased from 70 to 908. Activation of the surface was necessary toform films in which the bundles of nanotubes were dispersed well.
Doping nanotube networks. F4TCNQ (TCI America) was dissolved to aconcentration of 0.4 mmol in chloroform by bath sonication for 45 min to 1 h. Theresulting bright yellow solution was filtered using a syringe filter before use. Afterfabrication, carbon nanotube networks were covered with sufficient solution to coverthe sample surface. The solution was left to sit for 60 s, and excess was then removedby spinning at 3,000 r.p.m. for 40 s. Samples were left to air dry for at least 30 minbefore measuring.
Fabrication of capacitive arrays. PDMS substrates patterned with nanotubes werestretched to 25% before laminating with one another. We mixed and degassedEcoflex 0010 silicone elastomer (Smooth-On 0010, TFB Plastics, 1:1 base tocrosslinker by volume) and spread it (using a piece of PDMS membrane as a‘paintbrush’) over the surface of one of the patterned substrates. We oriented asecond substrate, face down, perpendicular to the first, and pressed down. Weexpelled air bubbles and excess Ecoflex by rolling using a roll of tape. We placeddrops of EGaIn (Aldrich) on one of the two exposed termini of each line, placed acopper wire in each drop of EGaIn, and embedded the EGaIn drops with additionalEcoflex. Curing at 100 8C for 1 h produced monolithic arrays of capacitivepressure sensors.
Electrochemical, optical and sheet-resistance measurements. We measuredresistance versus strain of single- and multipixel devices by clamping the device intoa purpose-built, programmable stage to apply tensile strain. We measured resistanceand capacitance using an LCR (inductance, capacitance, resistance) meter (AgilentE498A precision LCR meter) interfaced with a custom LabView script. We measuredcapacitance versus strain by applying compressive force perpendicular to the device,
aHigh-contrast
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Figure 5 | Images showing the characteristics of a 64-pixel array ofcompressible pressure sensors. a, Photograph of the device, with enhancedcontrast to show the lines of nanotubes (scale bar, 1 cm). b, Photograph ofthe same device reversibly adhered to a backlit liquid-crystal display. c, Mapof the estimated pressure profile over a two-dimensional area based on thechange in capacitance registered by a central pixel and its four nearestneighbours when a pressure of 1 MPa is applied to the central pixel (scalebar, 2 mm). d, Image of the device being deformed by hand.
with the device placed between a programmable vertically movable stage and a forcegauge (Mark-10 model BG05) with a probe (area of contact defined by a square cutfrom a glass slide). In all cases, we used EGaIn to form deformable electrical contactsto the stretchable nanotube films.
We measured the optical transmission of the nanotube films using a Cary 6000ispectrophotometer. The reported values of transmission were taken at 550 nm.
We obtained measurements of sheet resistance using four collinear, equallyspaced probes connected to a Keithley 2400 sourcemeter.
Received 7 September 2011; accepted 27 September 2011;published online 23 October 2011; corrected online 28 October 2011
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AcknowledgementsThis work was supported by a US Intelligence Community Postdoctoral Fellowship (toD.J.L.) and the Stanford Global Climate and Energy Program. B.C-K.T. was supported bythe Singapore National Science Scholarship from the Agency for Science Technology andResearch (A*STAR). The authors thank V. Ballarotto for helpful discussions and J.A.Bolander for writing code for the apparatus used for electromechanical measurements.
Author contributionsD.J.L. and Z.B. conceived the project. D.J.L., M.V. and B.C-K.T. performed and designedthe experiments. S.L.H. prepared the materials and developed the conditions used to dopethe nanotube films. J.A.L. deposited additional nanotube films. J.A.L. and C.H.F. performedexperiments on resistance versus strain. D.J.L., B.C-K.T., M.V., S.L.H. and Z.B. analysed thedata. D.J.L. wrote the paper. All authors discussed the results and commented onthe manuscript.
Additional informationThe authors declare no competing financial interests. Supplementary informationaccompanies this paper at www.nature.com/naturenanotechnology. Reprints andpermission information is available online at http://www.nature.com/reprints. Correspondenceand requests for materials should be addressed to Z.B.
